The invention relates to a method in a dryer section of a fiber-web machine, such as a paper or board machine, and to a device in such a machine.
As known from the prior art, drying groups of dryer sections based on cylinder drying in a fiber-web machine, such as a paper or board machine, employ twin-wire draw and/or single-wire draw. In twin-wire draw, the groups of drying cylinders comprise two wires, which press the web, one from above and the other one from below, against heated cylinder surfaces. In single-wire draw, each group of drying cylinders comprises only one drying wire on whose support the web runs through the entire group so that the drying wire presses the web against the heated cylinder surfaces of the drying cylinders and the web remains at the side of the outside curve of the reversing cylinders or rolls situated between the drying cylinders. Thus, in single-wire draw, the drying cylinders are arranged outside the wire loop, and the reversing cylinders or rolls are arranged inside the loop. In the following description, by the term ‘reversing cylinder’ is also meant a reversing roll alternatively placed in a corresponding position, and the terms ‘reversing cylinder’ and ‘reversing roll’ are used synonymously in this description. In the single-wire draw groups of dryer sections based on cylinder drying in fiber-web machines, a pocket space defined by a wire is formed between two adjacent drying cylinders and a reversing cylinder situated between them in a lower row. In connection with the pocket space, we speak of ‘opening’ and ‘closing nips’, i.e. ‘opening’ and ‘closing gaps’, by which opening nip, or opening gap, is meant an area where the drying wire separates from the drying cylinder and, correspondingly, a closing nip, or a closing gap, is formed on the side of the pocket space when the wire runs to the reversing cylinder. In a similar manner, when the wire leaves the reversing cylinder, an opening nip, i.e. an opening gap, is formed on the outgoing side of the pocket space and, correspondingly, when the wire runs to the next drying cylinder, a closing nip, i.e. a closing gap, is formed between the drying wire and the drying cylinder.
With increasing speeds of fiber-web machines, runnability problems have also begun to occur in the area of single-wire draw, in particular in the first drying cylinder groups of the dryer section. As known from the prior art, attempts have been made to reduce these problems by using different runnability components, such as, for example, blow boxes of the type marketed by Metso Paper, Inc. under the trademark HiRun. One runnability component of this type is described in FI patent 110625, which discloses a blowing device in a fiber-web machine or equivalent, which blowing device comprises a blow box which is arranged at the nip opening from the drying cylinder, i.e. at the opening gap, between the wire and the cylinder and which is provided with at least two nozzles arranged close to the wire. The first nozzle is arranged at the gap opening from the drying cylinder between the wire and the cylinder, for blowing air away from the gap between the wire and the blowing device. The second nozzle is arranged at a distance from said opening gap, in the running direction of the wire. The air jets discharging from the nozzles maintain a negative pressure in the space between the blowing device and the web. In the blowing device, at a short distance from said opening gap, there is further arranged a throttling member projecting toward the wire and dividing the negative pressure space formed between the first nozzle and the second nozzle into a first region of an intensified negative pressure confined to the location of the opening gap and into a second region of a lower negative pressure. An arrangement is also known from the state of the art in which there are mechanical seals on both sides of the opening gap and, in addition, there are suctions.
Seals placed in connection with blowing devices and equivalent are used in connection with runnability components for the purpose of confining desired negative pressure areas. If the tension of the drying wire is not correct, the wire may bend in the direction of the negative pressure of the runnability component and wear the seals of said component on the side of the wire, after which the runnability component no longer operates in the desired manner. This leads to loss of runnability and, at the same time, to a reduction of efficiency as the rate of production decreases, and the economical losses resulting from this can be really great.
Different arrangements are known from the state of the art in which attempts have been made to maintain the tension of the drying wire and, in particular, to prevent the wire from bending in the area of the gap opening from the drying cylinder and to prevent wear of the seals of the runnability component. The most common known arrangement to prevent the bending of the wire has been to increase the tension of the wire, for example, based on information provided by different wire-tension monitoring arrangements. One arrangement of this kind is described in FI patent application 20040048. On the other hand, attempts have been made to control the wire-bending problem, arising from the negative pressure caused by runnability components, in the area of the gap opening from the drying cylinder by lowering the level of the negative pressure. This, however, leads to runnability problems. The final solution for wear of the seals of negative-pressure components caused by the bending of the wire has naturally been to replace the seals with new ones until these wear again.
On the other hand, with respect to the state of the art, reference can be made to FI patent application 912416 and to FI laid-open publication 83345. These prior-art problem-solutions comprise arrangements in connection with a dryer section of a paper machine in which, in drying groups based on cylinder drying in the dryer section, the run between a drying cylinder and a reversing cylinder or roll in the area of an opening gap has been arranged to be short. By this kind of arrangement it is possible to improve to some extent the problems associated with the bending caused by too low wire tension but, on the other hand, it is problematic, among other things, because of shortage of space, to place runnability components known from the state of the art in connection with this kind of gap opening from a drying cylinder where the run to the next reversing cylinder is short.
With respect to the state of the art, reference can also be made to FI patent 77288, which describes an arrangement in which, in connection with a transition from a drying cylinder group applying single-wire draw to a drying cylinder group applying twin-wire draw, a special group-gap cylinder is used in the gap between the groups, and a suction tube device is arranged in connection with the group-gap cylinder, and in which the transfer from the last drying cylinder of the drying group applying single-wire draw to this special group-gap cylinder is arranged to be short.